Potential decoupling of CO2 and Hg uptake process by global vegetation in the 21st century

Mercury (Hg), a potent neurotoxin posing risks to human health, is cycled through vegetation uptake, which is susceptible to climate change impacts. However, the extent and pattern of these impacts are largely unknown, obstructing predictions of Hg’s fate in terrestrial ecosystems. Here, we evaluate the effects of climate change on vegetation elemental Hg [Hg(0)] uptake using a state-of-the-art global terrestrial Hg model (CLM5-Hg) that incorporates plant physiology. In a business-as-usual scenario, the terrestrial Hg(0) sink is predicted to decrease by 1870 Mg yr−1 in 2100, that is ~60% lower than the present-day condition. We find a potential decoupling between the trends of CO2 assimilation and Hg(0) uptake process by vegetation in the 21st century, caused by the decreased stomatal conductance with increasing CO2. This implies a substantial influx of Hg into aquatic ecosystems, posing an elevated threat that warrants consideration during the evaluation of the effectiveness of the Minamata Convention.

The manuscript describes the modeling findings on the change of carbon dioxide (CO2) and gaseous elemental mercury (Hg0) uptake by plants under several climate scenarios using the NCAR Community Land Model 5 with Hg processors (CLM5-Hg).The primary conclusion from the model results is that Hg uptake by vegetafion is expected to decrease significantly through 2100 under a business-as-usual climate scenario because of decreasing stomatal conductance with increasing concentrafion of CO2; and that global Hg uptake by vegetafion, which has been thought to be closely associated with the global carbon cycle, will be gradually "decoupled" from CO2 assimilafion caused by vegetafion.This is an interesfing study that aftempts to describe the long-term trend of an important sink term of atmospheric Hg(0) facilitated by foliar uptake in the terrestrial ecosystem.The work is relevant considering the concerted efforts devoted to source eliminafion of atmospheric Hg release regulated by the Minamata Convenfion of Mercury.However, it is not completely clear what scienfific processors are incorporated in the CLM5-Hg (not even in Yuan et. al., 2023, hftps://doi.org/10.1016/j.envint.2023.107904,where the model was described), making the results of such long-term simulafion doubfful and somewhat difficult to evaluate.Another technical concern is that the stated "Hg(0) uptake by vegetafion" is not clearly defined and the presented "preset-day uptake (3,138 Mg/yr)" appears to be much greater than what is currently understood.It is also unclear if the release and re-emission from the bare soil surface are included in the calculafion.Furthermore, since the biogeochemical cycle of Hg in vegetated ecosystems is sfill relafively poorly understood, biases in model results can propagate over a long simulafion period, which is the case in this work.These concerns, and the associated uncertainfies of the modeling framework, do not project confidence that the conclusion would be valid.Finally, there is a general lack of testable hypotheses in this work, and the study objecfives are not laid out specifically.
Overall, I recommend a major revision of the manuscript before considerafion for publicafion.Specific comments are as follows: 1.In the introducfion, provide the study hypotheses and the associated study objecfives to jusfify the need for this work.2. Clearly describe what processors are included in the CLM5-Hg model.What is equally important is what fluxes are being simulated.Is it only Hg(0) uptake by vegetafion, or the underlying soil release and re-emission are also included?What are the vegetafion types?What soil chemistry and surface redox chemistry are considered in the model?Are wet deposifion terms of Hg(II) included in the model?How are throughfall fluxes calculated?Is the immediate reflux of deposited Hg considered?Substanfial modeling details are missing in this work and Yuan et. al. (2023).Without understanding the parameterizafions, it is difficult to evaluate and verify the model results.3.In Figure 1a, what are the "uptake" terms included in the graph?It appears that there is substanfial "vegetafion uptake" in regions where there is no vegetafion.The reported value is much greater than what is currently known (e.g., about 1200 Mg/yr) based on lifterfall data.If throughfall is included, how is it esfimated given the general lack of through measurement for model verificafion?4. It is unclear how the elevated CO2 would lead to changes in the surface resistance terms (parficularly the stomatal resistance) and how it would lead to the decoupling claimed in the manuscript.A mechanisfic explanafion will benefit understanding of the underlying processes forced by the changed climate. 5.The trend shown in Figure 2, in addifion to the uptake quanfity in quesfion, does not appear to be consistent with the current understanding of the future trend of vegetafion Hg(0) uptake under a warming climate.A warming climate will lead to melfing of surface ice and permafrost soil.The vegetafive succession after the melfing and the increased intensity of precipitafion will lead to a substanfial increase of vegetafive biomass that increases the Hg(0) uptake by vegetafion.The model results are counter-intuifive and there is liftle plausible explanafion provided in the manuscript.6. Similarly in Figure 3, it is possible the land-cover changes under the changed climate would lead to significantly different vegetafion types and distribufion.Is the effect of landuse changes considered in the modeling?7.In the implicafion secfion, it is not clear how the "complex and extensive feedback mechanisms between the terrestrial Hg, water and C cycles" are illustrated in this work.The discussion can benefit from a more specific discussion on what processes are responsible for the simulated changes of the model results, and why the simulated results and the temporal trends appear to be significantly different from the assessment in earlier studies.
Reviewer #2 (Remarks to the Author): I liked that the authors of this modeling paper were very honest about the limitafions of their model calling this effort a diagnosfic exercise.They have provided a nice summary of how changes in CO2 concentrafions and climate change could impact Hg uptake.I feel they were very honest about the limitafions of this study.Some limitafions not addressed clearly include impacts of deforestafion that is only menfioned briefly in the implicafions secfion.They note that plants physiologically could adapt to higher concentrafions.Is there any work to indicate how plant behavior could change?They are clear that the experimental work was done with seedlings, and there will be differences associated with different plant species.Line 287 it has been demonstrated in mulfiple studies that Hg does not enter the plants via the roots and that roots act as a barrier accumulafing soil Hg Line 320 I would change this to read "bypass sequestrafion of Hgo by plants and deposifion of foliar Hg to the soil causing increasing concentrafions in the atmosphere, where it could be converted to HgII that ecosystems.It encompasses both stomatal and non-stomatal uptake in leaves, throughfall, soil formafion and leaching, photo-reducfion, microbial decomposifion, thermal evaporafion, and emissions from wildfires.In my assessment, the model's components allow for a comprehensive tracking of the Hg cycle involving land plants.Parficularly interesfing is the applicafion of disfinguishing numerous plant funcfional types (PFTs) characterized by different leaf, stem, and root properfies.This approach has been successfully tested against observafional field data in a previous study by the authors (Yuan et al., 2023).The data used regarding simulated atmospheric Hg(0) concentrafions, as well as the dry and wet deposifion fluxes of Hg(II), do not raise concerns, according to Zhang & Zhang (2022).Similarly, the ufilizafion of the global vegetafion Hg(0) flux dataset (Feinberg et al., 2022;Yuan et al., 2023), as well as CO2 dataset (Millhollen et al., 2006;Sañudo-Wilhelmy et al., 2008;Stamenkovic and Gusfin, 2009;Duval et al., 2011;Demers et al., 2013;Tang et al., 2021), is adequate.The uncertainty analysis appears to have been conducted thoroughly and comprehensively in my opinion.
In conclusion, the manuscript is presented in a clear and generally grammafically correct manner.However, I advise authors to carefully check through the text to ensure consistency in verb tenses, especially when discussing model results or future research direcfions.
Below, you will find a list of my suggesfions.

Introducfion
Lines 35-36: Consider adding informafion on the adverse effects of Hg on the ecosystem Lines 39-42: Consider adding informafion that long-living vegetafion can store not only present-day Hg but also Hg emifted into the atmosphere decades ago Line 42: Change "warming temperatures" to "rising temperatures".The same applies to line 51 Line 45: Add a comma after "vegetafion" Lines 51-52: The symbol "Hg" has already been explained at the beginning of the Introducfion Line 60: Correct the citafion for "Zhang et al., 2016" Line 74: What is the source of pre-industrial data?Also, correct "c.a" to "ca." Line 82: Change to "Materials and Methods" Results and Discussion Line 86: Consider specifying what kind of scenarios you are referring to, in this case "business-as-usual".The same applies to line 188 Lines 106-107: Consider adding "2°C scenario" and "regional rivalry".The same applies to line 213 Line 138: Provide a clear understanding of what the percentages represent, e.g., a change in Hg uptake or a decrease in foliage Hg levels Line 179: Change to "Decoupled CO2 and Hg" since you are referring to CO2 specifically rather than total carbon.The same applies to lines 232, 305, and 333 Lines 189-192: Consider providing a concise statement that summarizes the key finding before delving into the specific details.This can help guide the reader's understanding Lines 204-207: Consider breaking down this sentence into shorter ones for befter clarity.The same applies to lines 208-212 Lines 217-220: Consider providing a brief transifion sentence.Also, please explain "CO2 ferfilizafion effect" Lines 225-227: Please clarify whether you mean a decoupling in trends or a decoupling in the magnitude of the trends Lines 247-249: Please rewrite this sentence for befter clarity, e.g., "This occurs in part because the anomaly forcing method assumes that future changes (anomalies) can overlay present-day variability."Lines 252-253: Consider providing a brief transifion sentence, e.g., "Addifionally, it's important to note that some data sources in our meta-analysis originate from seedling experiments."Lines 271-274: Please rewrite this sentence for befter clarity.Also, please be consistent in terminology used, either "Medlyn" or "medlyn" Line 283-285: The statement lacks specificity.Please provide more details on the specific uncertainfies in the model representafion of land-atmosphere Hg exchange and its implicafions for future predicfions Lines 285-287: Please clarify the significance of these processes in the context of your study.How do they contribute to the uncertainfies menfioned earlier?Lines 287-288: Please explain the rafionale behind excluding the absorpfion of Hg from underground root systems and root secrefions.This omission could have implicafions for the overall accuracy of your model Line 288-289: Consider elaborafing on why anthropogenic and legacy Hg emissions remain unchanged.Provide jusfificafion or discuss potenfial impacts on the study's outcomes.Lines 317-319: Consider rephrasing the beginning for smoother flow, e.g., "Our findings reveal a suppression of atmospheric Hg

Materials and Methods
Lines 613-619: From what does the division of the soil pool into specific layers in the model result?Has this approach been applied before, and if so, could you please provide references?Lines 630-631: One of the model assumpfions is that anthropogenic mercury (Hg) will persist at the current level (Zhang et al., 2016).However, some esfimafions indicate that global anthropogenic emissions of Hg will increase in the forthcoming decades under the current legislafive scenario, e.g.  .Even if this rise won't substanfial compared to the present-day level, I believe it would be worthwhile to consider this aspect somewhere in the manuscript.
Reviewer #1 (Remarks to the Author): The manuscript describes the modeling findings on the change of carbon dioxide (CO2) and gaseous elemental mercury (Hg0) uptake by plants under several climate scenarios using the NCAR Community Land Model 5 with Hg processors (CLM5-Hg).The primary conclusion from the model results is that Hg uptake by vegetation is expected to decrease significantly through 2100 under a business-as-usual climate scenario because of decreasing stomatal conductance with increasing concentration of CO2; and that global Hg uptake by vegetation, which has been thought to be closely associated with the global carbon cycle, will be gradually "decoupled" from CO2 assimilation caused by vegetation.This is an interesting study that attempts to describe the long-term trend of an important sink term of atmospheric Hg(0) facilitated by foliar uptake in the terrestrial ecosystem.The work is relevant considering the concerted efforts devoted to source elimination of atmospheric Hg release regulated by the Minamata Convention of Mercury.However, it is not completely clear what scientific processors are incorporated in the CLM5-Hg (not even in Yuan et.al., 2023, https://doi.org/10.1016/j.envint.2023.107904,where the model was described), making the results of such long-term simulation doubtful and somewhat difficult to evaluate.Another technical concern is that the stated "Hg(0) uptake by vegetation" is not clearly defined and the presented "preset-day uptake (3,138 Mg/yr)" appears to be much greater than what is currently understood.It is also unclear if the release and re-emission from the bare soil surface are included in the calculation.Furthermore, since the biogeochemical cycle of Hg in vegetated ecosystems is still relatively poorly understood, biases in model results can propagate over a long simulation period, which is the case in this work.These concerns, and the associated uncertainties of the modeling framework, do not project confidence that the conclusion would be valid.Finally, there is a general lack of testable hypotheses in this work, and the study objectives are not laid out specifically.Overall, I recommend a major revision of the manuscript before consideration for publication.Response: Thank you very much for your recognition and detailed and constructive feedback on our manuscript.We appreciate the time and effort you have devoted to reviewing our work.Your comments have provided us with valuable insights that will undoubtedly improve the quality and clarity of our research.We have carefully considered each point you raised and have made corresponding revisions to address these concerns in the revised manuscript.Additionally, we have included diagrams and equations in the supplementary materials, as detailed in our responses to your specific comments below: Specific comments are as follows: 1.In the introduction, provide the study hypotheses and the associated study objectives to justify the need for this work.Response: We thank you for pointing out this issue.We have added the following sentences to the beginning of the last paragraph in the introduction.Lines 63-66 "Therefore, our research aims to examine how vegetation-regulated atmospheric Hg(0) deposition will change under the impact of future climate change.We hypothesize that future climate change will increase the global atmospheric Hg(0) deposition, as suggested by previous studies (Wang 2020a;Wu et al. 2012)." Additionally, we have added the following conclusive sentence at the beginning of the implications section.Lines 357-359: "We found that, in the climate change scenario, the atmospheric Hg(0) uptake by terrestrial vegetation in 2100 will be likely to decrease by more than half compared to present-day conditions."Without understanding the parameterizations, it is difficult to evaluate and verify the model results.Response: Thank you very much for this great suggestion, which we believe helps the readers to better understand our work.Our currently developed CLM5-Hg is a mechanistic model based on a relatively complete understanding of the Hg processes in terrestrial ecosystems, with the underlying soil release and re-emission already included (see Supplementary Fig. 1, as added below).However, our current investigation is primarily concentrated on vegetative uptake of Hg(0), since vegetation within terrestrial ecosystems absorbs gaseous elementary Hg [Hg(0)], serving as the largest removal mechanism of atmospheric Hg and the pivotal process within the terrestrial Hg cycle, given that vegetation typically responds more sensitively to climate change than soil does.Regarding the description of the mechanistic processes of CLM5-Hg that you mentioned, I will provide a detailed explanation for each point as follows: i.Specifically, the CLM model includes simulations for more than 15 types of Plant Functional Types (PFTs).However, due to the limitations of observational data under our eCO2 control experiment conditions, it is not feasible to subdivide PFTs for a direct one-to-one comparison.Therefore, in the validation process, we can only summarize the meta-sourced observational data according to their PFTs roughly and categorize them into three major groups: tree, grass, crop.Thus, referring to them as "vegetation type" was indeed inappropriate, and we have reverted to using "PFTs".The error has been corrected, and detailed descriptions have been added to the methods section to refine the corresponding part of the original manuscript: Lines 537-542: "Furthermore, due to the limitations of the data, it is not feasible to specifically subdivide PFTs for a direct one-to-one comparison between the model output and the observational data.Therefore, we have roughly categorized these studies into three subgroups: tree, grass, and crop (Table S2).This categorization aims to observe the impact of eCO2 on plants from different PFTs, enabling comparison with our model results."Line 152: "vegetation type" has been revised to "PFTs".
ii.We largely followed the Global Terrestrial Mercury Model (GTMM) by Smith-Downey et al. (2010) to simulate the soil chemistry and surface redox chemistry, with a few modifications.Compared with previous models, our model includes a vertically resolved soil biogeochemistry scheme.This scheme features base decomposition rates that vary with depth and are modified by soil temperature, water, and oxygen limitations.It also includes vertical mixing of soil carbon and nitrogen due to bioturbation, cryoturbation, and diffusion.The above supplementary content has been added to the revised Supplementary Information (SI) as follows: Lines 48-75 (SI): "The decomposition process of soil Hg is tied to the soil carbon pool, assuming that the Hg in the soil binds with soil carbon pools of different ages (Smith-Downey et al., 2010).The transformation between different Hg pools and the microbial transformation rate are characterized using the conversion and respiration rates of the soil carbon pool in every layer (Lawrence et al., 2019).Upon each transfer of carbon and Hg between pools, a fraction is lostcarbon as CO2 respiration and Hg as Hg(0) evasion into the atmosphere.Following microbial decay, we estimate that 16% of atmospheric Hg(0) evades, with the remainder being reincorporated into organic matter (Schaefer et al., 2020;Smith-Downey et al., 2010).In the framework of the singlelevel model structure, the foundational equation within decomposing Hg pools is as follows: (1) Where Ci is the Hg content of pool i, Ri represent the Hg inputs from plant tissues directly to pool i (only non-zero for litter pools and CWD), ki is the decay constant of carbon pool i; Tji indicates the fraction of carbon directed from pool j to pool i, with a fraction rj being lost as a respiration flux along the way.fHg is the constant representing the evasion into the atmospehere as Hg(0).
Incorporating the vertical dimension into the decomposition dynamics alters the balance equation as detailed below: (2) Where Ci(z) is now defined at each model level in volumetric terms (gC m -3 ), along with Ri(z) and kj(z).Additionally, vertical transport is accounted for by the last two terms, representing diffusive and advective transport, respectively.In the base model, advective transport is set to zero, leaving only a diffusive flux, with diffusivity D(z) defined for all decomposing carbon and Hg pools.Our model includes a vertically resolved soil biogeochemistry scheme in CLM5 was introduced.This scheme features base decomposition rates that vary with depth and are modified by soil temperature, water, and oxygen limitations.It also includes vertical mixing of soil carbon and nitrogen due to bioturbation, cryoturbation, and diffusion (Lawrence et al., 2019)." iii.Wet deposition of Hg(II) are also include in our model following the GTMM, and the monthly flux data are speciated from the CAM6-Chem-Hg (Zhang and Zhang, 2022).Additionally, we do not consider the immediate reflux of deposited mercury; instead, we focus on the gross uptake of Hg(0), although our model includes the re-emission process.The related supplementary content has been added to the revised SI and manuscript as follows: Lines 36-40 (SI): "The wet deposition process of Hg(II) is simulated following the GTMM (Smith-Downey et al., 2010).The Hg(II) wet deposition fluxes are speciated from the CAM6-Chem-Hg (Zhang and Zhang, 2022).After being removed from the surfaces of leaves and soil, Hg(II) can penetrate the soil and has the ability to attach to reduced sulfur groups present in organic matter.Throughfall flux are calculated as the sum of the Hg(II) dry deposition onto the canopy surface and the Hg(II) wet deposition that has not been reduced (Smith-Downey et al., 2010)."Lines 91-93: "This uptake represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes and does not include the immediate re-emission from foliage."Lines 108-110: "the value represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes and does not include the immediate re-emission from foliage."Lines 30-35 (SI): "The nonstomatal uptake pathway entails the direct absorption of atmospheric Hg vapor via the cuticles in the epidermis of the canopy's upper layers (Wesely, 1989).We assume a 10% re-emission of Hg from foliage due to the release and subsequent reduction of previously sequestered Hg(0) within leaf tissue, along with a 15% re-emission from Hg deposited on leaf surfaces, transformed to Hg(0) through photoreduction (Demers et al., 2013;Yu et al., 2016;Yuan et al., 2019)."Lines 337-346: "In our CLM5-Hg model, throughfall primarily originates from the washing off of atmospheric divalent mercury (sum of the Hg(II) dry deposition onto the canopy surface and the Hg(II) wet deposition that has not been reduced ) (Paige Wright et al., 2016;Smith-Downey et al., 2010).Recent studies indicate that epiphytic vegetation on canopies absorbs atmospheric Hg(0) and decomposes into humus, adhering to tree trunks and canopies, where mercury is subsequently washed into throughfall by precipitation (Wang et al., 2020b).Additionally, research indicates that the temporal scale and frequency of sampling for throughfall mercury measurements can impact the accuracy of their estimates (Choi et al., 2008).Therefore, our model has limitations in this part, and more extensive experimental data covering broader spatiotemporal scales is needed to further constrain the model(e.g., flux measurements or isotope compositions)."Supplementary Fig. 1 The terrestrial Hg cycle as simulated by CLM5-Hg, modified from Yuan et al. (2023).
3. In Figure 1a, what are the "uptake" terms included in the graph?It appears that there is substantial "vegetation uptake" in regions where there is no vegetation.The reported value is much greater than what is currently known (e.g., about 1200 Mg/yr) based on litterfall data.If throughfall is included, how is it estimated given the general lack of through measurement for model verification?Response: Thank you for pointing these out."uptake" refers to the dry deposition of Hg(0) driven by global vegetation.It represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes, and does not account for immediate reflux of foliage.Our simulation results agree well with the global vegetation distribution as follows: areas represented with values in the figure indeed correspond to actual vegetation presence (Southworth et al., 2023).I wonder if you might be questioning the presence of vegetation in tundra regions?Previous studies have shown that vegetation in tundra areas, such as mosses, can absorb atmospheric Hg(0) (Obrist et al., 2017;Wang et al., 2020a).The 1200 Mg/yr value based on litterfall data is very likely underestimated since these data primarily represent foliar Hg over the period near the growing season and do not account for the amount from the tissues, mosses, and lichens (Feinberg et al., 2022).Therefore, it cannot accurately estimate reversely to the Hg(0) vegetation uptake.This point was mentioned in our previous study (Yuan et al., 2023).
In our CLM5-Hg model, throughfall flux are calculated as the sum of the Hg(II) dry deposition onto the canopy surface and the Hg(II) wet deposition that has not been reduced.It is partly refer to our response to previous comments.The above-mentioned content has been added to the revised manuscript as follows: Lines 91-93: "This uptake represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes and does not include the immediate re-emission from foliage."Lines 108-110: "a, Hg(0) vegetation uptake flux at present-day, the value represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes and does not include the immediate re-emission from foliage."Lines 337-346: "In our CLM5-Hg model, throughfall primarily originates from the washing off of atmospheric divalent mercury (sum of the Hg(II) dry deposition onto the canopy surface and the Hg(II) wet deposition that has not been reduced ) (Paige Wright et al., 2016;Smith-Downey et al., 2010).Recent studies indicate that epiphytic vegetation on canopies absorbs atmospheric Hg(0) and decomposes into humus, adhering to tree trunks and canopies, where mercury is subsequently washed into throughfall by precipitation (Wang et al., 2020b).Additionally, research indicates that the temporal scale and frequency of sampling for throughfall mercury measurements can impact the accuracy of their estimates (Choi et al., 2008).Therefore, our model has limitations in this part, and more extensive experimental data covering broader spatiotemporal scales is needed to further constrain the model(e.g., flux measurements or isotope compositions)" 4. It is unclear how the elevated CO2 would lead to changes in the surface resistance terms (particularly the stomatal resistance) and how it would lead to the decoupling claimed in the manuscript.A mechanistic explanation will benefit understanding of the underlying processes forced by the changed climate.Response: Regarding the process by which elevated CO2 leads to changes in leaf stomatal resistance (stomatal conductance), we elaborated in detail in the section 'Decoupled CO2 and Hg': Lines 219-227: "The Medlyn model in our CLM5-Hg model is consistent with this optimal stomatal theory.With eCO2, the CO2 partial pressure at the leaf surface (CS) also increases accordingly, leading to enhanced leaf photosynthesis (An) (equation ( 1)).However, An is constrained by water potential while Cs continues to increase in CLM5 (Lawrence et al., 2019).This resulted in reduced stomatal conductance (gs) with eCO2 (Supplementary Fig. 13)…This adaptive mechanism ultimately leads to a nonlinear relationship between atmospheric CO2 concentration and stomatal conductance." As for the decoupling claimed in our study, we elaborated it at the end of the section.We also added descriptions related to the trade-offs between the effects by increasing LAI and reducing gs (stomatal conductance) to the original text.The integrated explanation is as follows: Lines 250-263: Contrarily, we predicted a decoupling between the trends of CO2 assimilation and Hg(0) uptake by vegetation in the 21st century.The CLM5 model projects an increased greening of vegetation in many regions in the 21st century resulting from eCO2 (a.k.a.fertilization effect), evidenced by the increased leaf area index (LAI) in the northern mid-to-low latitudes and certain regions of the Southern Hemisphere (Fig. 4c).The increase in photosynthesis can simultaneously induce a state of water deficit and nutrient saturation within the plant's internal environment (Victoria et al., 2010).Therefore, under climate change, the increase in vegetation LAI may only represent an increase in leaf density or even stomatal numbers, but stomatal conductance may not necessarily increase accordingly.Our model suggests a discernible decrease in the flux of Hg(0) uptake by vegetation in these areas (Fig. 1), reflecting the differences in CO2 and Hg elements during plant physiological processes especially those related to water dynamics in terrestrial ecosystems (Wang et al., 2021).
5. The trend shown in Figure 2, in addition to the uptake quantity in question, does not appear to be consistent with the current understanding of the future trend of vegetation Hg(0) uptake under a warming climate.A warming climate will lead to melting of surface ice and permafrost soil.The vegetative succession after the melting and the increased intensity of precipitation will lead to a substantial increase of vegetative biomass that increases the Hg(0) uptake by vegetation.The model results are counter-intuitive and there is little plausible explanation provided in the manuscript.Response: We appreciate the reviewer for pointing out the issues related to a warming climate and for providing many excellent and constructive suggestions.We completely agree that it is very likely that vegetative succession following melting and increased precipitation intensity will lead to an increase in vegetative biomass and, consequently, an increase in Hg(0) uptake by vegetation.Our study did not consider the impacts of Land Use and Land Cover Change (LULCC), because we focused more on the direct effects of climate change (e.g.eCO2) in this research.This point is very insightful, and our next work is addressing this aspect.We clearly stated this point in the revised manuscript as follows: Lines 86-87: "…and remove the effects of land use and land cover change (LULCC) and aerosols (see Materials and Methods)."Lines 385-390: "We did not consider the impacts LULCC in this study.Under global warming, vegetative succession following melting and increased precipitation intensity is likely to lead to an increase in vegetative biomass and, consequently, an increase in Hg(0) uptake by vegetation (Wang et al., 2020a).Indeed, the interactive effects of climate change combined with changes in LULCC worth further examination."6. Similarly in Figure 3, it is possible the land-cover changes under the changed climate would lead to significantly different vegetation types and distribution.Is the effect of landuse changes considered in the modeling?Response: The results presented in Figure 3 are also based on OAAT analysis focusing solely on the influence of CO2.In this study, we didn't considered the effects of LULCC, please refer to our response to last comment and Lines 480-482: "To eliminate the impact of land use and land cover changes, as well as aerosols, we preprocessed the vegetation patterns and aerosol deposition in all simulation scenarios using the specified file provided by CLM5."Your suggestion is very valuable, and in future research, we could explore the interactive effects of LULCC and climate change, which could be an intriguing study."7.In the implication section, it is not clear how the "complex and extensive feedback mechanisms between the terrestrial Hg, water and C cycles" are illustrated in this work.The discussion can benefit from a more specific discussion on what processes are responsible for the simulated changes of the model results, and why the simulated results and the temporal trends appear to be significantly different from the assessment in earlier studies.Response: Thank you for pointing this out.The sentence was originally intended to summarize the results and discussion sections from before, and placing it here was indeed somewhat inappropriate.Therefore, we have moved it to Line 215 of the "Decoupled CO2 and Hg" section in the revised manuscript.In this section, we thoroughly discuss what processes are responsible for the simulated changes in the model results and why our findings differ significantly as follows: Lines 215-263: "Our study illustrates a complex and extensive feedback mechanism between the terrestrial Hg, water and carbon cycles.The enhancement of photosynthesis caused by the biogeochemical effect of eCO2 is accompanied by the loss of plant water content (Katul et al., 2012).During this process, plants adjust the stomatal aperture to reduce water transpiration and maximize water use efficiency (Gardner et al., 2023;Hsiao et al., 2019).The Medlyn model in our CLM5-Hg model is consistent with this optimal stomatal theory.With eCO2, the CO2 partial pressure at the leaf surface (CS) also increases accordingly, leading to enhanced leaf photosynthesis (An) (equation ( 1)).However, An is constrained by water potential while Cs continues to increase in CLM5 (Lawrence et al., 2019).This results in reduced stomatal conductance (gs) with eCO2 (Supplementary Fig. 13), leading to decreased transpiration (Fig. 4d).This process induces increased soil water storage, enhancing water use efficiency (Supplementary Fig. 14).…A tight coupling between carbon and Hg in terrestrial ecosystems has been observed as a paradigm over the past two decades (Smith-Downey et al., 2010;Wang et al., 2020a).Conventionally, it has been postulated that rising atmospheric CO2 levels would increase vegetation's photosynthesis rate, leading to the beneficial impact on plant growth, known as the CO2 fertilization effect (Liu et al., 2019).This effect is believed to enhance the concurrent absorption of both CO2 and Hg(0), as suggested by Jiskra et al. (2018), Lawrence et al. (2019), Obrist (2007), and Schaefer et al. (2020).…Contrarily, we predict a decoupling between the trends of CO2 assimilation and Hg(0) uptake by vegetation in the 21st century, when considering the dynamic response of vegetation physiological activities to climate change.…The increase in photosynthesis can simultaneously induce a state of water deficit and nutrient saturation within the plant's internal environment (Victoria et al., 2010).Therefore, under climate change, the increase in vegetation LAI may only represent an increase in leaf density or even stomatal numbers, but stomatal conductance may not necessarily increase accordingly.However, our model suggests a discernible decrease in the flux of Hg(0) uptake by vegetation in these areas (Fig. 1), reflecting the differences in CO2 and Hg element during plant physiological processes especially those related to water dynamics in terrestrial ecosystems (Wang et al., 2021)." Reviewer #2 (Remarks to the Author): I liked that the authors of this modeling paper were very honest about the limitations of their model calling this effort a diagnostic exercise.They have provided a nice summary of how changes in CO2 concentrations and climate change could impact Hg uptake.I feel they were very honest about the limitations of this study.Response: Thank you very much for your positive and constructive feedback on our manuscript.We are grateful for your recognition of our research findings and our transparency of the model limitations.Please refer to our response to specific comments as follows: 1.Some limitations not addressed clearly include impacts of deforestation that is only mentioned briefly in the implications section.Response: Thank you for bringing this point up.In this study, our focus is on the effect of climate change on vegetation Hg(0) uptake.We haven't considered the effects of land use and land cover (LULCC), as described in the Methods section of the manuscript: Lines 481-483: "To eliminate the impact of land use and land cover (LULCC) changes, as well as aerosols, we preprocessed the vegetation patterns and aerosol deposition in all simulation scenarios using the specified file provided by CLM5."Your suggestion is very valuable, and in our future research, we plan to explore the interactive effects of LULCC and climate change, which could be an intriguing study.The added sentences in the revised manuscript are as follows: Lines 386-391: "We did not consider the impacts LULCC in this study.Under global warming, vegetative succession following melting and increased precipitation intensity is likely to lead to an increase in vegetative biomass and, consequently, an increase in Hg(0) uptake by vegetation (Wang et al., 2020a).Indeed, the interactive effects of climate change combined with changes in LULCC worth further examination."Lines 86-87: "…and remove the effects of land use and land cover change (LULCC) and aerosols (see Materials and Methods)."2. They note that plants physiologically could adapt to higher concentrations.Is there any work to indicate how plant behavior could change?Response: Thank you for bringing this point up.It is reported that under the influence of longterm eCO2 conditions, stomatal density and size will decrease, which are attributed to the guard cells and mesophyll tissues that mediate stomatal movements.The improved sentences have been added to the revised manuscript as follows: Lines 233-239 : "Intriguingly, the sensitivity of stomata to eCO2 diminishes gradually under the influence of long-term eCO2 conditions.This occurs because guard cells and mesophyll tissues, which mediate stomatal movements, lead to decreases in stomatal aperture and size, culminating in physiological adaptation to higher concentrations (Engineer et al., 2016;Liang et al., 2023).Consequently, this results in a less pronounced decline from the SSP3-7.0 to SSP5-8.5 scenarios (Fig. 2)." 3.They are clear that the experimental work was done with seedlings, and there will be differences associated with different plant species.Response: Thanks for your good suggestion.We have added and improved the following sentences as follows: Lines 285-288: "Additionally, there will be differences associated with different plant species.Thus, we suggest that future research should focus on this aspect, aiming to bridge the knowledge gap by including experiments across various growth stages and more species." 4. Line 287 it has been demonstrated in multiple studies that Hg does not enter the plants via the roots and that roots act as a barrier accumulating soil Hg Response: Thank you for pointing this out.As you mentioned, Hg is hard to enters the plants via the root, as most previous studies have shown (Arnold et al., 2018;Chiarantini et al., 2016;Siwik et al., 2010).Regarding this, we have made the following supplementary explanation in the revised manuscript: Lines 327-336: "Our model also did not consider the absorption of Hg from underground root systems and root secretions (Keuper et al., 2020).Indeed, Hg is hard to enter the plants via the root, as most previous studies have shown (Arnold et al., 2018;Chiarantini et al., 2016;Siwik et al., 2010).Meanwhile, our model does not account for the translocation of Hg among plant tissue organs.A recent study suggests that a significant proportion of Hg in roots may originate from absorption by leaves and subsequent translocation, with an estimation of up to 300 Mg yr -1 of atmospheric Hg⁰ stored in roots (Zhou et al., 2021), but the specific migration and distribution mechanisms are still unclear.These processes also have a potential influence on the amount of Hg(0) uptake by the vegetation, and could be incorporated in our model when more data is available.5. Line 320 I would change this to read "bypass sequestration of Hgo by plants and deposition of foliar Hg to the soil causing increasing concentrations in the atmosphere, where it could be converted to HgII that is deposited to ecosystems, where it can subsequently be methylated.Gaseous elemental Hg is the form taken up by foliage.HgII is not taken up see Arnold et al. (2018) Response: Thank you very much for the valuable suggestion.We have rewritten the sentences as advised, as follows: Lines 370-373: With climate change, the bypassing of atmospheric Hg(0) sequestration by plants and the deposition of foliar Hg to the soil lead to increasing concentrations in the atmosphere.This Hg can then be converted to HgII, which is deposited in aquatic ecosystems and can subsequently be methylated (Arnold et al., 2018;Feinberg et al., 2022;Zhou et al., 2021).Some general housekeeping 6. Line 28 which should be that.Response: Revised as suggested 7. Line 48 previous research has Response: Revised as suggested 8. Line 84 and throughout the text what was done should be described in past tense.Here indicated should be used.Line 89 were instead of are.Response: Thanks for your suggestions.We have thoroughly checked and corrected all the tenses that needed to be changed, and the changes have been highlighted in revised manuscript.9. References for Supplemental Information are not in alphabetical order and Arnold J, Gustin MS, Weisberg PJ.Evidence for Nonstomatal Uptake of Hg by Aspen and FiguresFigure1:Please add in the capfion the unit of fime (yr-1) for the global total values of Hg(0) uptake.In line 101 change to "Materials and Methods" Figure2: Please add explanafions to the scenario numbers in the legend.Figure 3: What does the purple triangle on Figure 3a mean? Figure 4: It seems there is an error in the color scale legend of figures 4a and 4c.Now it is 0 -3 -2 -1 instead of 0 -1 -2 -3

Figure 3 :
FiguresFigure1:Please add in the capfion the unit of fime (yr-1) for the global total values of Hg(0) uptake.In line 101 change to "Materials and Methods" Figure2: Please add explanafions to the scenario numbers in the legend.Figure 3: What does the purple triangle on Figure 3a mean? Figure 4: It seems there is an error in the color scale legend of figures 4a and 4c.Now it is 0 -3 -2 -1 instead of 0 -1 -2 -3 2. Clearly describe what processors are included in the CLM5-Hg model.What is equally important is what fluxes are being simulated.Is it only Hg(0) uptake by vegetation, or the underlying soil release and re-emission are also included?What are the vegetation types?What soil chemistry and surface redox chemistry are considered in the model?Are wet deposition terms of Hg(II) included in the model?How are throughfall fluxes calculated?Is the immediate reflux of deposited Hg considered?Substantial modeling details are missing in this work and Yuan et.al. (2023).